Chlorine dioxide is a gas of known utility. Recently, the use of chlorine dioxide gas for chemosterilizing surfaces, especially gas impermeable surfaces of implements commonly employed in the medical sciences, including surfaces formed from porcelain, ceramics, metals, plastics and glass has been disclosed, for example in U.S. Pat. Nos. 4,504,442 and 4,681,739 (Rosenblatt et al.).
There are known methods for producing chlorine dioxide gas. In one of these methods, sodium chlorite is reacted with chlorine gas in a chlorine dioxide generating apparatus. When the chlorite is in solid form, the chlorine dioxide generating apparatus can be, for example, a conventional column, or a bed filled with the chlorite.
In U.S. Pat. No. 2,309,457 (Hutchinson et al.), chlorine gas diluted with an inert gas such as air or nitrogen is brought into contact with a bed or column of solid chlorite and caused to react therewith to form chlorine dioxide and the corresponding chloride. The chlorite is preferably in flaked or finely-divided form. The chlorine may be dry or may contain water vapor. Similarly, the chlorite may be dry or contain about 5% moisture. The chlorine dioxide produced is said to be substantially free from chlorine when the gas feed rate and the supply of fresh chlorite is properly adjusted.
U.S. Pat. No. 4,689,169 (Mason et al.) describes a dry composition for producing chlorine dioxide gas consisting of a dry inert diluent such as sodium chloride, a metal chlorite such as sodium chlorite, and a dry oxidizing agent as chlorine or a dry acidic agent such as citric acid, which is capable of reacting with the metal chlorite in the dry state to produce chlorine dioxide. The chlorine dioxide gas produced by the reaction of dry oxidizing or acidifying agent with dry sodium chlorite is mixed with the inert gas and a certain amount of unreacted chlorine gas.
U.S. Pat. No. 2,043,284 (Cunningham et al.) describes a process in which chlorine is reacted with an aqueous solution of sodium chlorite. Due to its chemical instability, the chlorine is diluted with an inert gaseous diluent such as nitrogen or air to produce chloride dioxide that is similarly diluted, in order to keep the concentration of chlorine dioxide within safe limits.
The chlorine dioxide gas produced by methods such as those discussed above may then be used for its desired purpose, or may be subjected to further treatment prior to use to remove any chlorine gas which may also be present. Since chlorine dioxide is sometimes used in operations in which the presence of even a small amount of chlorine renders it unsuitable (e.g., because chlorine is highly corrosive), it is often desirable to remove chlorine gas from the chlorine/chlorine dioxide gas mixture.
Methods are known in the art for separating chlorine from the gas mixture. For example, U.S. Pat. No. 2,036,375 (Vincent) describes a method whereby the gas mixture including chlorine is contacted with the oxides, hydroxides, and various carbonates of magnesium, the alkali metals and the alkaline earth metals in substantially dry form. U.S. Pat. No. 2,131,447 (Logan) describes a method of preparing chlorine dioxide which is said to be completely free of chlorine by causing a regulated quantity of chlorine to react with a chlorite solution in a separate vessel removed from the stripping effect of the inert gas.
The chlorine dioxide gas which may or may not have been further treated as above thereafter may be fed to a further apparatus for use. Once the chlorine dioxide gas has been used for its intended purpose, it must be eliminated in some manner.
Effluent gas containing the chlorine and/or chlorine dioxide should not be directly released to the atmosphere. After use, gaseous mixtures containing chlorine or chlorine dioxide should be subjected to further treatment. Chlorine and/or chlorine dioxide may be absorbed or adsorbed by other chemicals in order to reach maximum allowable OSHA exposure limits (for chlorine and chlorine dioxide, these are 1 and 0.1 ppm, respectively, for an 8-hour time waited average exposure).
Efforts have been made to remove chlorine and/or chlorine dioxide from an effluent gas prior to release of the effluent gas into the atmosphere. Witches brew (which includes sodium hydroxide and potassium salts) has been used as a scrubbing solution but is problematic because it is a variable mix and thus unreliable, subject to pH and thermal excursions, and produces undefinable reactant products which may pose serious waste disposal concerns.
Sodium thiosulfate solution has been used as scrubbing agent, but has drawbacks. The reaction produces sulfuric acid, which reaction is exothermic, thus potentially making the solution hot. This heating may result in a variety of unpredictable side reactions forming a potentially broad range of undesirable reaction products. The reaction also causes the solution to become acidic, which reverses the reaction and causes the sodium chlorite product to revert to chlorine dioxide. Other solutions considered for use as scrubbing liquors for chlorine dioxide removal in bleach plant waste gases include sulfur dioxide solution, hypochlorite solution, caustic solution, and water. Japanese Patent No. 75,139,077 (1975; Aoki, Y.) describes the removal of chlorine and chlorine dioxide from waste gases by countercurrent scrubbing with a white liquor consisting of NaOH (59.3 kg.m.sup.3), Na.sub.2 CO.sub.3 (14.8 kg/m.sup.3), Na.sub.2 S (25.5 kg/m.sup.3) Na.sub.2 SO.sub.4 (0.2 kg/m.sup.3) in the ratio of 100 parts waste gas to 1 part liquor. Japanese Patent No. 7,557,883 (1975; Mitsuta, H.) describes the removal of chlorine dioxide by exchanging the gases in a column filled with active carbon granules sprayed with water at the top.